1. Field of the Invention
The present invention relates to a projector and more particularly, though not exclusively, to a projector that modulates a light beam via a reflective liquid-crystal panel.
2. Description of the Related Art
Reflective liquid-crystal panels have a higher aperture ratio and higher definition than transmissive liquid-crystal panels, and therefore, projectors equipped with reflective liquid-crystal panels are in great demand. However, in contrast to transmissive-type projectors, reflective-type projectors have the two following optical problems, which have been setting back the popularization of reflective-type projectors.
One of the problems is that a reflection of image light produced in an optical projecting system re-enters the reflective liquid-crystal panel where the light is reflected again so as to re-enter the optical projecting system. As a result, the light passes through the optical projecting system to reach the screen, causing the image contrast to decrease. The other problem is that the light needs to be selected depending on whether the reflective liquid-crystal panel is in an ON mode (a mode in which light is projected) or an OFF mode (a mode in which light is not projected), a polarizing beam splitter must be disposed in front of the panel. For this reason, the color separating/combining system tends to become large in size.
As an attempt to solve the first problem, U.S. Pat. No. 5,268,775 and U.S. Pat. No. 5,786,873 disclose examples in which the direction of polarization of feedback light is rotated by about 90° (e.g., by using a quarter-waveplate) so that the feedback light is substantially removed by a polarizing beam splitter and a polarizer.
On the other hand, for the purpose of solving the second problem, US AA2002/0140905 discusses a system in which two polarizing beam splitters are provided with respect to three reflective liquid-crystal panels corresponding to three primary colors, such that color separation and color combination are performed in the two polarizing beam splitters. According to US AA2002/0140905, wavelength-selective polarization rotators are provided proximate an incident side and an exit side of each polarizing beam splitter. This facilitates the control of the ON-OFF mode of two of the reflective liquid-crystal panels by one of the polarizing beam splitters, so as to select whether to project or not to project light.
However, simply combining these two examples does not solve the two aforementioned problems.